Find the chilled or heating water flow rate needed to carry a thermal load at your design temperature difference.
For the electrical side of the same plant, use the main kVA to kW converter.
Difference between flow and return
Every diagram and table on this page updates with the calculator above.
Convert kW to BTU/hr first by multiplying by 3,412.142.
The 500 constant is water doing the work: 8.33 lb per gallon × 60 minutes per hour × 1 BTU per lb·°F. Change the fluid and the constant changes with it.
In SI units the same relationship is L/s = kW ÷ (4.18 × ΔT in °C), where 4.18 is the specific heat of water in kJ/kg·K.
Note: Assumes plain water; glycol mixtures need a correction factor.
Flow rate and temperature rise are the two ways to move a given quantity of heat. Fix the load and they trade off exactly — double the temperature difference and you halve the flow.
That single relationship drives the whole hydraulic design: pipe diameters, pump sizes, valve authority and the energy the pumps will consume for the next twenty years.
| System | Typical design ΔT | Notes |
|---|---|---|
| Chilled water, conventional | 10 – 12 °F (5.5 – 6.7 °C) | Standard for most commercial systems |
| Chilled water, high delta-T | 16 – 20 °F (9 – 11 °C) | Smaller pipes and pumps, needs careful coil selection |
| Heating, radiators | 20 °F (11 °C) | Traditional design |
| Heating, condensing boiler | 36 °F (20 °C) | Low return temperature keeps the boiler condensing |
| District heating | 40 – 90 °F (22 – 50 °C) | Large ΔT minimises network pumping |
Pumping power scales roughly with the cube of flow, so halving the flow can cut pump energy by around 85%. That is why raising design delta-T is one of the highest-value decisions in a hydraulic system.
Required water flow across a range of loads at your design temperature difference.
| kW | Water flow (GPM) | Water flow (L/s) |
|---|---|---|
| 5 kW | 3.4 | 0.22 |
| 10 kW | 6.8 | 0.43 |
| 20 kW | 13.6 | 0.86 |
| 35 kW | 23.9 | 1.51 |
| 50 kW | 34.1 | 2.15 |
| 75 kW | 51.2 | 3.23 |
| 100 kW | 68.2 | 4.31 |
| 150 kW | 102.4 | 6.46 |
| 200 kW | 136.5 | 8.61 |
| 300 kW | 204.7 | 12.92 |
| 500 kW | 341.2 | 21.53 |
| 700 kW | 477.7 | 30.14 |
| 1,000 kW | 682.4 | 43.05 |
The clearest way to see what a design decision on delta-T costs or saves in flow rate.
| kW | 8 °F | 10 °F | 12 °F | 16 °F | 20 °F |
|---|---|---|---|---|---|
| 10 kW | 8.5 | 6.8 | 5.7 | 4.3 | 3.4 |
| 25 kW | 21.3 | 17.1 | 14.2 | 10.7 | 8.5 |
| 50 kW | 42.7 | 34.1 | 28.4 | 21.3 | 17.1 |
| 100 kW | 85.3 | 68.2 | 56.9 | 42.7 | 34.1 |
| 200 kW | 170.6 | 136.5 | 113.7 | 85.3 | 68.2 |
| 400 kW | 341.2 | 273 | 227.5 | 170.6 | 136.5 |
| 800 kW | 682.4 | 545.9 | 455 | 341.2 | 273 |
Once flow is known, pipe size follows from a velocity limit. Too fast causes noise and erosion; too slow means oversized pipe and poor air removal.
| Nominal size | Typical flow at 1.5 m/s | Approximate |
|---|---|---|
| DN25 (1") | 0.7 L/s | 11 GPM |
| DN40 (1½") | 1.9 L/s | 30 GPM |
| DN50 (2") | 3.0 L/s | 48 GPM |
| DN80 (3") | 7.5 L/s | 119 GPM |
| DN100 (4") | 12 L/s | 190 GPM |
| DN150 (6") | 27 L/s | 428 GPM |
Indicative only — final sizing depends on the velocity and pressure drop limits in your design standard.
When the measured temperature difference falls below design, the system pumps more water than necessary without delivering extra cooling. It is one of the most common faults in commercial chilled water plant.
Bypass flow mixes return water back into the loop, diluting the return temperature.
Dirty or under-performing coils cannot extract the design temperature rise.
Over-pumped circuits push water through too fast for full heat transfer.
29 calculators built on the same maths as the tool above. Each one has its own inputs, interactive diagrams and worked examples.
Common questions about converting kw to gpm.